In a communication system, important modulation modes include quadrature phase shift keying (Quadrature Phase Shift Keying, QPSK), 16 quadrature amplitude modulation (16 Quadrature Amplitude Modulation, 16QAM), and dual polarization quadrature phase shift keying (Dual Polarization Quadrature Phase Shift Keying, DP-QPSK). Accordingly, a QPSK signal may be obtained by adopting the QPSK modulation, a 16QAM signal may be obtained by adopting the 16QAM modulation, and a DP-QPSK signal may be obtained by adopting the DP-QPSK modulation.
Referring to FIG. 1, FIG. 1 is a schematic diagram of an existing modulation structure for generating a QPSK signal. As shown in FIG. 1, a QPSK modulator receives an externally input data stream (Data) and an optical signal that is input by a laser diode (LD), and performs QPSK modulation on the input optical signal by using the data stream, so as to obtain a QPSK signal and output the QPSK signal. The QPSK signal modulation structure show in FIG. 1 is capable of generating only a QPSK signal, but cannot generate other signals.
Referring to FIG. 2, FIG. 2 is an existing schematic diagram of a modulation structure for generating a 16QAM signal. As shown in FIG. 2, an optical signal output by an LD is split into a first optical signal and a second optical signal that have the same optical amplitude (that is, having the same optical power), and the second optical signal is processed by a variable optical attenuator (VOA), so that the optical amplitude of the second optical signal is attenuated to 25% (that is, the optical power is attenuated to 25%); a first QPSK modulator receives externally input Data and the first optical signal, and performs QPSK modulation on the first optical signal by using the externally input Data, so as to obtain a first QPSK signal; a second QPSK modulator receives externally input Data and the second optical signal that is processed by the VOA, and performs QPSK modulation on the second optical signal that is processed by the VOA by using the externally input Data, so as to obtain a second QPSK signal; and a 16QAM signal is obtained by combining the first QPSK signal and the second QPSK signal, and is then output. The 16QAM signal modulation structure shown in FIG. 2 is capable of generating only a 16QAM signal, but cannot generate other signals.
Referring to FIG. 3, FIG. 3 is a schematic diagram of an existing modulation structure for generating a DP-QPSK signal. As shown in FIG. 3, an optical signal of an LD is input into a polarization beam splitter (PBS) and then split into a first optical signal and a second optical signal that have different polarization directions; the polarization direction of the first optical signal is orthogonal to that of the second optical signal, and the optical power of the first optical signal is equal to that of the second optical signal; a first QPSK modulator receives externally input Data and the first optical signal, and performs QPSK modulation on the first optical signal by using the externally input Data, so as to obtain a first QPSK signal; a second QPSK modulator receives externally input Data and the second optical signal, and performs QPSK modulation on the second optical signal by using the externally input Data, so as to obtain a second QPSK signal; and a DP-QPSK signal is obtained by combining the first QPSK signal and the second QPSK signal through a polarization beam combiner (PBC), and is then output. The DP-QPSK signal modulation structure shown in FIG. 3 is capable of generating only a DP-QPSK signal, but cannot generate other signals.
The modulation structures for generating the QPSK, 16QAM, and DP-QPSK signals are capable of generating only a corresponding type of signals respectively, but cannot generate other types of signals.